1. Field of the Invention
The present invention relates to visualization of parts having metallic objects against a dielectric or non-metallic poorly contrasting background. The invention is particularly, but not exclusively, useful for recognizing by computer vision a Ball Grid Array (BGA) of small solder balls carried by a light colored ceramic substrate for the purposes of inspection and surface mounting the BGA on a circuit board or card.
2. Description of the Related Art
Typical Ball Grid Arrays (BGAs) are described in IBM Application Note, xe2x80x9cCeramic Ball Grid Array Surface Mount Assembly and Reworkxe2x80x9d, July 2000.
In such publication, at page 10, it is indicated that in the case of BGA packages employing a white ceramic substrate, poor lighting contrast between the solder balls and the white ceramic substrate may make it difficult to tune a vision system, causing failure to properly recognize BGA packages and erroneous rejections.
A BGA or Ceramic Ball Grid Array (CBGA) is a commonly used component or package in Surface Mount Technology (SMT). The bottom of the part is formed by high melt small (e.g. a fraction of a mm in diameter) solder balls arranged in a predetermined, typically rectangular grid array, pattern on a ceramic substrate to which they are joined by eutectic solder fillets. The part is placed on a circuit board or card that has a matching pattern of contact pads on which eutectic solder paste has been applied. The circuit board or card is then run through an oven such that the solder paste forms fillets joining the solder balls to the contact pads.
Currently, a pick and place machine is used to pick the BGA, along with other parts, out of a storage location, and computer vision equipment, including an upwardly looking camera and frontal lighting, is employed by the pick and place machine to inspect the BGA, and if the BGA is not rejected, to steer the placement of the BGA on the circuit board or card with proper registration of the BGA balls to the circuit pads of the board or card. The BGA should be rejected if it has one or more missing balls, incorrect ball spacing, or nonsymmetrical ball size.
Most BGAs have a dark colored ceramic bottom and silver colored metallic balls. This combination provides excellent contrast between balls and background. Some BGAs have silver or gold colored metallic balls on a white or very light colored ceramic background. These parts do not provide sufficient visual contrast for current computer vision systems to reliably recognize the BGA. Specifically, when recognition using current camera and lighting is done on BGAs having a white or light colored bottom, the vision system cannot distinguish between balls and background because the balls appear almost as bright as the ceramic background.
One known technique used to attempt to improve the visualization and, as a result the recognition, of white ceramic BGAs, involves illuminating the part being visualized by so-called xe2x80x9ccoaxial lightingxe2x80x9d wherein the illuminating light is introduced via a beam splitter in front of the camera lens so that the resulting illumination is coaxial with the lens of the camera. The use of such coaxial lighting has proven to be marginally effective at best. When coaxial lighting is used in combination with a rather expensive telecentric camera lens, a higher, but still substandard, level of successful recognition is achieved.
It is an object of the present invention to overcome the aforementioned deficiencies in the prior art visualization of metallic objects against a poorly contrasting dielectric background, and in particular to improve the visualization, and thus the rate of successful recognition, of white or light colored ceramic BGAs.
This and other objects of the present invention are satisfied by a method and apparatus in which the part being visualized is illuminated by linearly polarized electromagnetic radiation, e.g light passed through a polarization filter, and an image is formed of the electromagnetic radiation reflected from the part and passed through a polarization filter oriented to pass radiation having a polarization direction that is substantially orthogonal to the direction of polarization of the radiation illuminating the part. Because the radiation is reflected from the metallic objects or solder balls with the same polarization direction as the illuminating radiation, substantially no radiation reflected from the metallic objects passes through the latter polarization filter and appears in the formed image. On the other hand, because radiation reflected from the non-metallic or dielectric background is smeared in polarization and includes radiation with a polarization direction that is orthogonal to the polarization direction of the illuminating radiation, some radiation reflected from the background passes through the latter polarization filter and appears in the formed image. Consequently, the part is visualized in the formed image with a substantial contrast between metallic objects and background, e.g. the metallic objects appear black, whereas the background appears white or light colored, allowing for reliable computer vision recognition of the metallic objects.
A method in accordance with the invention for visualization of a part having metallic objects against a light colored non-metallic background comprises illuminating the part with electromagnetic radiation that is linearly polarized in a predetermined first direction, and forming an image of electromagnetic radiation reflected from the part viewed through a linear polarization filter oriented for passing electromagnetic radiation that is linearly polarized in a second direction that is substantially orthogonal to the first polarization direction. Such method further comprises recognizing the metallic object in the formed image.
Similarly, an apparatus in accordance with the present invention for visualization of a part having metallic objects against a light colored non-metallic background comprises one or more sources for illuminating the part with electromagnetic radiation that is linearly polarized, at least one of the sources producing electromagnetic radiation that is linearly polarized in a predetermined first direction, and an image forming device for forming an image of electromagnetic radiation reflected from the part viewed through a linear polarization filter oriented for passing electromagnetic radiation that is linearly polarized in a second direction that is substantially orthogonal to the first direction. Such apparatus further comprises a computer vision system for recognizing the metallic objects in the formed image, and still further comprises a manipulator for positioning the part on a circuit board or card with recognized metallic objects of the part in registration with contact pads of the board or card.
The invention also comprises a circuit board or card on which is surface mounted a part that has been recognized in accordance with the aforementioned method, such surface mounting being with recognized metallic objects of the part in registration with contact pads of the board or card.
Other objects, features and advantages of the present invention will become apparent upon perusal of the following detailed description when taken in conjunction with the appended drawing, wherein: